My friend and I are having a disagreement regarding the behaviour of
sailboats on a mooring. We both have limited sailing experience, although he
has a fair amount more than I. We were discussing running up a spinnaker at
mooring in calm air, just to practice putting it up. We eventually
concluded that this wasn't practical, but the argument came in as to our our
different conclusions as to why it would not be. I felt that there would be
no point to put it up, as it would just get blown into the rigging and not
really tell us anything and possibly get torn. He was of the opinion that
somehow the spinnaker could push the boat to one side, fill and turn the
boat around. Initially he said it would go right upwind of the mooring,
then reduced his claim to say that it would turn around at the point where
it was attached to the line at the bow.

I am of the opinion that this is physically impossible and that under no
combination of sail at a mooring could the boat turn around so that the aft
was upwind. I am of the opinion that you could get the boat to swing to one
side or the other a bit by backing the jib, but you could still never turn
the boat around.

I would gratefully accept as many informed opinions on this from a sailing
or physics point of view. As this letter has generated extremely divergent
threads in other groups, please let me say that the question I ask is "in
the case of a regular fractional rig sloop, ignoring tidal issues, is it
possible for a spinnaker, or any combination of sail to move the aft around
to windward while on a bow attached mooring?". Letting out enough line to
be effectively not moored is not a legitimate option.